This invention relates to electron gun assemblies and more particularly to electron gun assemblies of the type used in color television picture tubes.
In an in-line electron gun assembly having a structure comprising three separate cathodes, a control grid (also referred to as grid No. 1) spaced from the cathodes, and a screen grid (also referred to as grid No. 2) spaced from the control grid, separate bias voltages are applied to the cathodes. These bias voltages are adjusted to provide simultaneous cutoff of the beam currents for black level adjustment. Grid No. 1 is normally at zero volts and an adjusted value of grid No. 2 voltage is provided to establish the cathode cutoff bias voltages in a range of approximately 100 to 150 volts.
In a typical setup for operating the tube, video drive signals of the proper levels are applied to the cathodes so as to track from black level to all levels of standard white picture throughout the useful picture dynamic range. For quality tube operation, it is desirable that this cutoff setup of the three guns be kept in equal cutoff relation, one to the other, so that white picture tracking is maintained.
In the prior art, the desired equality of cutoff relationship has not been maintained during the warm-up period, which is usually considered to include approximately the first 15 minutes after the filament has been turned on. This inequality occurred because the cathode-to-grid No. 1 spacings of the three guns vary differently as the cathode and related structures are heated. Since the cathode-to-grid No. 1 spacing is considered to be the most important factor in establishing cutoff, there must be equality of expansion in time and magnitude if all three cathode-to-grid No. 1 spacings are to be maintained in unison.